Some electrified transportation systems, such as electric vehicles (EV) and plug-in hybrid electric vehicles (HEV), are moving towards drivetrains with high-power electric machines and inverters. These drivetrains often require energy storage systems with high voltage, high efficiency, and long lifetime (see, for example, A. Affanni, A. Bellini, G. Franceschini, P. Guglielmi, and C. Tassoni, “Battery choice and management for new-generation electric vehicles”, IEEE Trans. Ind. Electron., vol. 52, pp. 1343-1349, 2005; and S. M. Lukic, J. Cao, R. C. Bansal, F. Rodriguez, and A. Emadi, “Energy storage systems for automotive applications”, IEEE Trans. Ind. Electron., vol. 55, pp. 2258-2267, 2008, the entirety of which are incorporated herein by reference).
High voltage energy storage systems may be achieved by connecting energy storage elements, such as battery and/or supercapacitor cells, in series. The capacity of the stacked cells (i.e. the capacity of the individual energy storage elements) may differ due to internal (internal impedance and different self-discharge rate) and external (temperature variations) effects (see, for example, J. Cao, N. Schofield, and A. Emadi, “Battery balancing methods: A comprehensive review”, in Vehicle Power and Propulsion Conference (VPPC), 2008, the entirety of which is incorporated herein by reference). This may result in differences in charging and discharging of cells, which may cause charge imbalances across the stack that can worsen over time.
A battery management system (BMS) may check each cell individually for over and undercharging. Otherwise, the cell life may be reduced nearly exponentially with the string length (see, for example, N. H. Kutkut, H. L. N. Wiegman, D. M. Divan, and D. W. Novotny, “Charge equalization for an electric vehicle battery system”, IEEE Transactions on Aerospace and Electronic Systems, vol. 34, pp. 235-246, 1998; and “Design considerations for charge equalization of an electric vehicle battery system”, IEEE Trans. Ind. Appl., vol. 35, pp. 28-35, 1999, the entirety of which are incorporated herein by reference). If the state-of-charge (SOC) of the individual energy storage cells is not equalized (i.e. balanced), the stack may contain cells with both low and high SOC. This can lead to only a fraction of the energy storage capacity being accessible.